1. Technical Field
Embodiments of this disclosure generally relate to a fixing device to fix an unfixed toner image, and to an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having two or more of copying, printing, and facsimile functions, employing an electrophotographic system and incorporating the fixing device.
2. Related Art
Image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction machines having two or more of copying, printing, and facsimile functions usually incorporate a fixing device employing an electromagnetic induction heating method to reduce startup time of the image forming apparatuses incorporating the fixing device, thereby saving energy. For example, JP-2006-350054-A discloses such a fixing device using the electromagnetic induction heating method. The fixing device includes, e.g., a support roller (or a heating roller) serving as a heat generator, an auxiliary fixing roller (or a fixing roller), a fixing belt stretched over the support roller and the auxiliary fixing roller, an induction heater, serving as an induction heating unit, facing the support roller via the fixing belt, and a pressing roller to contact the auxiliary fixing roller via the fixing belt. The induction heater includes, e.g., a coil (or an excitation coil) wound in a longitudinal direction of the induction heater, and cores (or excitation coil cores) facing the coil. The induction heater faces and heats the fixing belt. The heated fixing belt heats and fixes a toner image on a recording medium conveyed at a fixing nip formed between the auxiliary fixing roller and the pressing roller.
Specifically, when a high-frequency alternating current is supplied to the coil, an alternating magnetic field formed around the coil generates eddy currents on a surface of the support roller and its neighboring area. When the eddy currents are generated around the support roller, the electric resistance of the support roller leads to Joule heating of the support roller, thereby heating the fixing belt stretched over the support roller.
In such a fixing device employing the electromagnetic induction heating method, a heat generator is directly heated by electromagnetic induction. Accordingly, compared to a fixing device using a halogen heater, such a fixing device employing the electromagnetic induction heating method has a higher heat-exchange efficiency and therefore the surface temperature of the fixing belt can be increased to a desired fixing temperature with reduced energy and a shorter startup time.
However, the electromagnetic induction heating method has difficulty in uniformly heating a heat generator in a longitudinal direction thereof because of the following two reasons. One reason is the behavior of eddy currents in the heat generator, and more specifically, for example, variation of the behavior of eddy currents caused by the shape of coil. In the process of the electromagnetic induction heating, eddy currents are generated in the heat generator by magnetic flux arising from the coil serving as a magnetic flux generator, and releases heat (i.e., Joule heating). Thus, the heat generator generates heat. The eddy currents basically follow the shape of a coil disposed in an induction heater.
Specifically, if the coil disposed facing the heat generator has only a straight part, the eddy currents travel in a linear manner. Accordingly, the heat generator is heated in a substantially uniform manner. However, in practice, the coil is turned somewhere. Typically, end portions of the heat generator correspond to turning parts of the coil, and the eddy currents traveling in the end portions of the heat generator differ from the eddy currents traveling in a middle portion of the heat generator. Accordingly, the heat distribution of the heat generator is not uniform in the longitudinal direction thereof.
The other reason is the shape of coil.
The induction heater heats the heat generator by the magnetic flux arising from the coil serving as a magnetic flux generator. Accordingly, if the magnetic flux arising from the coil is uniform in the longitudinal direction of the heat generator, the heat generator can be heated in a substantially uniform manner. However, as described above, the coil is turned somewhere in practice. The magnetic flux interlinking the heat generator is different at the end portions of the heat generator corresponding to the turning parts of the coil and at the middle portion of the heat generator. Accordingly, the heat distribution of the heat generator is not uniform in the longitudinal direction thereof.
Because of the above-described two reasons, a typical fixing device employing the electromagnetic induction heating method has a problem such that a heat generator used therein does not uniformly generate heat in a longitudinal direction thereof.
JP-2009-014972-A provides, e.g., an end core that covers an end of an excitation coil in a longitudinal direction thereof, thereby enhancing efficiency of heat generation by a heat generator. However, the shape of such an end core is relatively complicated, and moreover the end core is connected to another core.